Version 4.1 - 1.6.b NHRP
To start - NHRP is Next Hop Resolution Protocol. NHRP is an "ARP-like" protocol that dynamically maps the physical address to the logical address on NBMA networks. NBMA is Non-Broadcast Multi-Access.
When you think about this, visualize a hub and spoke. The hub is the Next Hop Server (NHS) and the Spokes are the Next Hop Clients (NHC). Remember that this is used in DMVPN. Also remember that the spokes may or may not know their IP address in advance.
The best way to understand NHRP is to look at the Configuration Examples in the documentation. Like the GRE section before it, this is easier to understand and discuss when talking about specific implementations. Basically, you configure a NHS. Then you configure the clients (NHC) with information to reach the server. The NHC registers with the NHS and the NHS learns the mapping information of the spoke. Once the Hub-to-Spoke tunnel is built, the NHS now has the information to let the NHCs know how to build spoke-to-spoke tunnels.
One of the better docs for NHRP is under IP Addressing: NHRP Configuration Guide. Under the section "How to Configure NHRP," they start by configuring a GRE tunnel. Note that the tunnel key is more of an identifier than a security feature. "The tunnel ID key is carried in each GRE packet, it is not carried in any NHRP messages. We do not recommend relying on this key for security purposes. " It also discusses configuring a NHRP network ID. This is a local-only parameter that is used to differentiate between multiple NHRP domains or networks. But keep reading. Later in the doc, it covers "Specifying the NHRP Authentication String." That was new to me. "Configuring an authentication string ensures that only routers configured with the same string can communicate using NHRP." Granted, the string is not encrypted and is not true authentication, but it is good to know that it exists.
If you give these links (above) a quick read, you will probably understand more about NHRP than you need to for the exam. If this information is already familiar to you, scan the docs quickly and move on.